Omnidirectional slot antenna

Abstract: There is provided a small-sized and simplified horizontally polarized antenna apparatus which forms an omnidirectional pattern in the horizontal plane. The radiation field in the horizontal plane becomes continuous and a horizontally polarized omnidirectional radiation pattern can be obtained in the horizontal plane by forming radiation slots at opposing positions on a grounded hollow body and exciting the slots out of phase.

Description: The present invention relates to a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane, and to a transponder provided with such an antenna apparatus. Figs. 1(a) and 1(b) schematically illustrate a configuration of a horizontal polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane explained in Chapter 12 of "VHF Antenna" written by Uchida and Mushiake, and issued by the Production Technology Center (March, 1977). Fig. 1(a) is a perspective view and Fig. 1(b) is a top plan view with electric field distribution indicated by arrows. In these figures, the numeral 50 designates a dipole antenna and the symbol I indicates a current flowing through the dipole. Next, operations will be explained. A grounded conductor 51 includes four surfaces and a dipole antenna 50 is arranged at each surface. The dipole antenna 50 is arranged in parallel to the horizontal surface to excite a horizontally polarized wave. A plurality of dipole antennas may be arranged in the vertical direction. Amplitudes of currents flowing through the dipole antennas in the same height are equal, but phases thereof are sequentially different by 90 degrees. A dipole antenna 50 has a figure-of-8 type radiation directivity, but substantially horizontally polarized omnidirectivity can be obtained through a combination of the four dipole elements. Figs. 2(a) - 2(c) show a conventional slot antenna indicated in "X-band omnidirectional double-slot array antenna" by T. Takeshima, ELECTRONIC ENGINEERING, No. 39, pp. 617-621 (October, 1967). These figures schematically illustrate a configuration of a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane (rectangular waveguide slot antenna). Fig. 2(a) is a perspective view, Fig. 2(b) is a sectional view along the line A-A and Fig. 2(c) is a side elevation. In Figs. 2(a) - 2(c), numeral 60 designates a radiation slot; 61 a waveguide; and 62 a flange. The principle in operation of the rectangular waveguide slot antenna shown in Figs. 2(a) - 2(c) will be explained with reference to Figs. 3(a) and 3(b). Fig. 3(a) is a diagram illustrating a distribution of magnetic field inside the waveguide 61. Fig. 3(b) is a cross-sectional view along the line A-A illustrating a distribution of magnetic field inside the waveguide and a current flowing along the side surface. Such distributions of magnetic field and current as illustrated in Figs. 3(a) and 3(b) can be realized by short-circuiting the end portions of the waveguide. Electromagnetic waves propagated along the rectangular waveguide 61 excite the radiation slots 60 to radiate electromagnetic waves if the radiation slots 60 are provided in parallel with the waveguide axis at the positions offset from the center of the H plane of the rectangular waveguide 61. In this case, the radiation slots 60 are excited by providing each of the radiation slots 60 at a position where the magnetic field inside the waveguide 61 becomes maximum. An amount of electromagnetic wave radiation can be adjusted by changing the position of each radiation slot 60. In order that the waveguide slot antenna shown in Figs. 2(a) - 2(c) may be used as a horizontally polarized omnidirectional antenna, the radiation slots 60 are provided, as shown in Fig. 4(a), on the front and rear H planes of the waveguide 61. Then, a distribution of electric field in the horizontal plane changes as shown in Fig. 4(b). The radiation slots 60 are excited out of phase and the radiation field becomes continuous in the horizontal plane. As a result, a theoretically omnidirectional directivity can be realized. However, if, as shown in Fig. 2(a), two radiation slots are formed symmetrically on the front and rear surfaces, two radiation slots can be excited in the same phase by arranging the radiation slots in symmetrical positions of the waveguide 61 with respect to the center thereof at an interval of λg/2 (λg is a wavelength in the waveguide). Therefore, a vertically symmetrical pattern can be obtained in the direction of  ±90° (in Fig. 4(a)), while a beam tilt is generated in the direction of  = 90° + α and  = 0° and 180° in Fig. 4(a) due to an array factor of the radiation field of the two radiation slots. Accordingly, on the x-y plane, a gain difference is generated in the direction of  = ±90°, 0° and 180° and a ripple in the horizontal plane becomes significant whereby no omnidirectivity can be achieved. In the case where one radiation slot is provided in a position offset from the center of the H plane of the waveguide, no symmetrical configuration is formed and actually no omnidirectivity can be obtained. Fig. 5 schematically illustrates a configuration of a transponder 70 provided with an antenna 71 shown in Fig. 2(a). This transponder 70 is provided with a transmitter/receiver (transceiver) 72 connected to the horizontally polarized antenna 71 which has an omnidirectional pattern in the horizontal plane. In an emergency such as an accident, the transceiver 72 is activated by turning a switch 73 ON, getting the transceiver ready for receiving a signal. When the transceiver 72 under this condition receives a radar signal radiated from a searching plane, the transceiver 72 is switched to an electromagnetic wave radiation mode and transmits a response signal. Thus, a person who has met with an accident can inform his position by generating an emergency signal and await rescue by a searching plane. The transceiver 72 is connected to a battery 74 and the transponder 70 is covered with a radome 75. An existing horizontally omnidirectional antenna structured such as explained above is widely used as an antenna apparatus for TV and radar. However, if a dipole antenna as shown in Fig. 1(a) is used, the apparatus itself has protrusions having large volumes and there is a difficulty in fixing the antenna and wiring power supply cables. If a waveguide slot antenna as shown in Fig. 2(a) is used, a substantially omnidirectional pattern can easily be achieved by providing radiation slots on the waveguide, but, if a ripple in the horizontal plane becomes large, any omnidirectional pattern cannot be obtained. Meanwhile, a conventional transponder has the following problems in practical use. First, it is necessary to place the transponder in a waiting mode by turning ON the switch, but, in an emergency case, a user sometimes forgets to turn ON the power switch. In this case, the transponder does not function, thereby endangering a user's life. Moreover, the transceiver, which normally transmits a signal upon reception of a radar signal radiated from a searching plane, has no means for indicating which condition the apparatus is in. For example, it is unclear whether the transceiver is sometimes inoperative and does not perform transmission even when the switch is turned ON. In addition, it is also impossible to detect, while a signal is transmitted from the transponder, whether a searching plane is coming closer. US-A-4,922,259 discloses an antenna in which two patch antenna elements are disposed in parallel to respective ground planes and are excited in opposite circular polarizations. GB 2,221,577 discloses a blade antenna with a similar arrangement of elements. US 3,969,730 discloses a slot type antenna formed by bolting together two right-angle plates each having cut-outs in its edges so that a square hollow antenna with slots extending around its right-angle edges is formed. The present invention has been proposed to overcome the problems described above and it is therefore an object of the present invention to provide a small-sized horizontally polarized omnidirectional antenna having a simplified configuration. Moreover, it is a further object of the present invention to provide a transponder comprising an omnidirectional horizontally polarized antenna and capable of notifying an operator who has issued an emergency signal that the apparatus is activated and in a waiting mode, that the apparatus is then in a transmission mode and that a searching plane is coming closer. An antenna apparatus having radiation slots arranged at opposite positions on a grounded conductive hollow body and a single signal feeding line entering said antenna to excite said slots with a signal, said grounded conductive hollow body comprising a set of parallel conductive plates each including one of said radiation slots, said feeding line and slots being arranged to excite said slots out of phase to form an omnidirectional radiation pattern in a plane perpendicular to said hollow body, strip lines being provided for connecting said signal feeding line to said parallel conductive plates; characterised in that said strip lines and parallel conductive plates constitute a triplate line exciting said radiation slots. The hollow body may be rectangular hollow body formed of two pairs of conductive plates. Since the slots are excited out of phase, the electrical field radiated from the radiation slots becomes continuous in a plane perpendicular to the hollow body, for instance, in the horizontal plane and therefore an omnidirectional radiation pattern can be obtained in the horizontal plane. The hollow body may be filled with a dielectric material whereby the antenna apparatus can be manufactured in a small size due to a wavelength shortening effect of the dielectric material. It is possible to make a through-hole in the dielectric material between the radiation slots. Since the radiation slots become longer to thereby resonate at the same frequency, a beam width becomes narrow in the plane perpendicular to the hollow body and a gain can be increased. A plurality of radiation slots may be provided along the longitudinal axis of the hollow body. In this case, the radiation slots formed on the opposing conductive plates are excited out of phase and the radiation slots formed on the same conductive plate are excited in phase. Consequently, a beam width in a plane including the longitudinal axis can be narrowed and a gain can be increased. In this case, a difference in length of signal feeding lines used to feed the adjacent radiation slots on the same conductive plate can be set to integer times an operating wavelength or odd number of times a half of the operating wavelength. It is possible to provide horn-type conductive plates on the conductive plates perpendicular to the longitudinal axis of the hollow body. The horn-type conductive plates enable a beam width in a plane including the longitudinal axis to be reduced without changing the size and position of the radiation slots and an omnidirectional high-gain radiation pattern to be achieved in the plane perpendicular to the longitudinal axis. Semi-cylindrical conductive plates may be provided to the conductive plates which have no radiation slots, whereby any influence of waves diffracted at the edges of the conductive plates can be avoided, an amount of ripple in the plane perpendicular to the longitudinal axis can be adjusted and an omnidirectional radiation pattern can be obtained without changing size and position of the slots. The signal feeding lines can be provided to the outer surfaces of dielectric layers formed on the opposing conductive plates. The present invention will be further described by way of example with reference to the accompanying drawings, in which:- Fig. 1(a) is a perspective view of a conventional omnidirectional antenna apparatus. Fig. 1(b) is a plan view of the antenna apparatus of Fig. 1(a), illustrating a distribution of electric field. Fig. 2(a) is a perspective view illustrating another conventional omnidirectional antenna apparatus. Fig. 2(b) is a cross-sectional view taken along the line A-A of Fig. 2(a). Fig. 2(c) is a side elevation of the antenna apparatus of Fig. 2(a). Fig. 3(a) illustrates a distribution of magnetic field in the antenna apparatus of Fig. 2(a). Fig. 3(b) illustrates directions of current and magnetic field at the cross-section taken along the line A-A of Fig. 3(a). Fig. 4(a) is a diagram for explaining directivity of the antenna apparatus of Fig. 2(a). Fig. 4(b) illustrates a horizontal distribution of electric field established by the antenna apparatus of Fig. 4(a). Fig. 5 is a partially cutout diagram illustrating a conventional transponder. Fig. 6(a) is a perspective view of a first embodiment of an antenna apparatus of the present invention. Fig. 6(b) is a cross-sectional view taken along the line A-A of Fig. 6(a). Fig. 6(c) is a cross-sectional view taken along the line B-B of Fig. 6(a). Fig. 7 is a diagram for explaining operations of the antenna apparatus of Fig. 6(a). Fig. 8 is a graph illustrating a gain in the azimuth direction of the antenna apparatus of Fig. 6(a). Fig. 9(a) is a perspective view of a second embodiment of an antenna apparatus of the present invention. Fig. 9(b) is a cross-sectional view taken along the line A-A of Fig. 9(a). Fig. 9(c) is a cross-sectional view taken along the line B-B of Fig. 9(a). Fig. 10(a) is a perspective view of a third embodiment of an antenna apparatus of the present invention. Fig. 10(b) is a cross-sectional view taken along the like A-A of Fig. 10(a). Fig. 10(c) is a cross-sectional view taken along the line B-B of Fig. 10(a). Fig. 11(a) is a perspective view of a fourth embodiment of an antenna apparatus of the present invention. Fig. 11(b) is a cross-sectional view taken along the line A-A of Fig. 11(a). Fig. 11(c) is a cross-sectional view taken along the line B-B of Fig. 11(a). Fig. 12(a) is a perspective view of a fifth embodiment of an antenna apparatus of the present invention. Fig. 12(b) is a cross-sectional view taken along the line A-A of Fig. 12(a). Fig. 12(c) is a cross-sectional view taken along the line B-B of Fig. 12(a). Fig. 13(a) is a perspective view of a sixth embodiment of an antenna apparatus of the present invention. Fig. 13(b) is a cross-sectional view taken along the line A-A of Fig. 13(a). Fig. 13(c) is a cross-sectional view taken along the line B-B of Fig. 13(a). Fig. 14(a) is a perspective view of a seventh embodiment of an antenna apparatus of the present invention. Fig. 14(b) is a cross-sectional view taken along the line A-A of Fig. 14(a). Fig. 15(a) is a perspective view of an eighth embodiment of an antenna apparatus of the present invention. Fig. 15(b) is a side elevation of the antenna apparatus of Fig. 15(a). Fig. 16(a) is a perspective view of a ninth embodiment of an antenna apparatus of the present invention. Fig. 16(b) is a cross-sectional view taken along the line A-A of Fig. 16(a). Fig. 16(c) is a side elevation of the antenna apparatus of Fig. 16(a). Fig. 17 is a perspective view of a radome employing an antenna apparatus of the present invention. Fig. 18 is a perspective view of a transponder utilizing any one of the first to ninth embodiments of the antenna apparatus of the present invention. In the drawings, the same numerals designate similar or corresponding elements. Embodiment 1 Figs. 6(a) - 6(c) schematically illustrate a configuration of the first embodiment of the present invention, Fig. 6(a) being a perspective view, Fig. 6(b) cross-sectional view taken along the line A-A of Fig. 6(a) and Fig. 6(c) a cross-sectional view taken along the line B-B of Fig. 6(a). In these figures, radiation slots 1, 1' are formed respectively on a first set of parallel conductive plates 2, 2' and both conductive plates 2, 2' are connected by a second set of conductive plates 3', 3", 3"' to configurate a rectangular parallelepiped. The inside of the rectangular parallelepiped is filled with a dielectric material 4. The radiation slots 1, 1' are excited by a triplate line 6 formed of the conductive plates 2, 2' and strip lines 5. Numeral 7 designates a coaxial connector for feeding the triplate line; and 8 a coaxial line. The conductive plates 2, 2', 3, 3', 3", 3"' are grounded. Fig. 7 is a diagram explaining the principle of the antenna apparatus of Fig. 6(a). A signal propagating through the coaxial line 8 enters the triplate line 6 via the coaxial connector 7. The triplate line 6 can be formed in a small size resulting in reduction in size of the antenna apparatus by filling the rectangular parallelepiped with the dielectric material 4. Both ends of the triplate line 6 are connected respectively to the right side edge of the radiation slot 1 and the left side edge of the slot 1' with respect to Fig. 6(b) and a voltage is applied across the strip line 5 and the first set of the ground conductive plates 2, 2'. Since the ends of the triplate line 6 are connected to the opposite side edges of the radiation slots 1, 1', the electric fields inside the rectangular parallelepiped formed of the first set of conductive plates 2, 2' and the second set of conductive plates 3', 3", 3"' are reversed with each other as indicated by the arrow marks in Fig. 7. Therefore, the radiation slots 1, 1' provided on the grounded conductive plates 2, 2' are excited out of phase (in a phase difference of 180 degrees). The radiation field formed by these radiation slots 1, 1' becomes continuous in the horizontal plane (azimuth direction) and a horizontally polarized omnidirectional radiation pattern can be obtained. In the first embodiment, the radiation slots 1, 1' are fed with the triplate line 6, but another feeding line such as a coaxial line can also be used for the same purpose. Fig. 8 indicates measured gains of horizontally polarized and vertically polarized waves when the antenna apparatus of Fig. 6(a) is rotated 360 degrees in the horizontal plane. As seen from Fig. 8, in the case of the horizontally polarized wave, an amount of ripple is within 2 dB, resulting in a substantially omnidirectional pattern. The gain of the vertically polarized wave which is a cross-polarized wave is -20 dB or less and a satisfactory characteristics results. Embodiment 2 Figs. 9(a) - 9(c) schematically illustrates a configuration of the second embodiment of the present invention, Fig .9(a) being perspective view, Fig. 9(b) a cross-sectional view taken along the line A-A and Fig. 9(c) a cross-sectional view taken along the line B-B. The second embodiment is different from the first embodiment in that both ends of the triplate line 6 are connected respectively to left side edge of the radiation slot 1 and the right side edge of the slot 1' with respect to Fig. 9(b). A voltage is applied across the radiation slots 1, 1' from the triplate line 6 for exciting the radiation slots 1, 1'. Since the radiation slots 1, 1' provided on the first set of grounded conductive plates 2, 2' are excited out of phase, a radiation field generated by these radiation slots 1, 1' becomes continuous in the horizontal plane (azimuth direction) and a horizontally polarized omnidirectional radiation pattern can be obtained. In this embodiment, the ends of the triplate line 6 are connected to the radiation slots 1, 1', but a similar characteristic can also be obtained by open-circuiting the ends of the triplate line and setting the length between the open-circuited ends and the radiation slots 1, 1' to approximately a quarter of the wavelength of an operating frequency. Embodiment 3 Figs. 10(a) - 10(c) schematically illustrate a configuration of the third embodiment of the present invention, Fig. 10(a) being a perspective view, Fig. 10(b) a cross-sectional view taken along the line A-A and Fig. 10(c) a cross-sectional view taken along the line B-B. This embodiment is different from the first embodiment in that a portion 9 of the dielectric material 4 corresponding to the radiation slots 1, 1' is removed. The antenna apparatus of this embodiment also shows, with the same principle as the antenna apparatus of the embodiment 1, a horizontally polarized omnidirectional radiation pattern. Since the portion 9 of the dielectric material 4 between the radiation slots 1, 1' formed on the first set of grounded conductive plates 2, 2' is removed, the radiation slots 1, 1' of the third embodiment must be longer, in order to have them resonate at the same resonance frequency than those of the first embodiment wherein no dielectric material 4 is removed, because a wavelength shortening effect by the dielectric material 4 is lost. The radiation slots 1, 1' being set longer, the beam width becomes narrow, the gain in the direction perpendicular to the plates 2, 2' increases and the gain in the horizontal plane can be increased. It is noted that a dielectric material may be provided in a parallelepiped defined by the radiation slots 1, 1'. Embodiment 4 Figs. 11(a) - 11(c) schematically illustrate a configuration of the fourth embodiment of the present invention, Fig. 11(a) being a perspective view, Fig. 11(b) a cross-sectional view taken along the line A-A and Fig. 11(c) a side elevation. In these figures, the strip lines 5, 5' are provided on second dielectric materials 11, 11' formed on the conductive plates 2, 2' so that microstrip lines 10, 10' are configurated by the first set of conductive plates 2, 2' and the strip conductors 5 and 5'. Next, operations will be explained. Ends of the microstrip lines 10 and 10' are open-circuited. At the ends the electric field is maximum, while the magnetic field is minimum. Since the magnetic field becomes maximum at a position separated a quarter of the wavelength from the ends of the microstrip lines, the radiation slots 1, 1' are electromagnetically coupled with the microstrip lines 10, 10' by providing such radiation slots 1, 1' at the position described above. Since the radiation slots 1, 1' provided on the first set of conductive plates 2, 2' are excited by the microstrip lines 10, 10' out of phase, the radiation field produced by the radiation slots 1, 1' becomes continuous in the horizontal plane (azimuth direction) and a horizontally polarized omnidirectional radiation pattern can be obtained. In the fourth embodiment, the ends of the microstrip lines 10, 10' are open-circuited to excite the radiation slots 1, 1', but the end of each microstrip line 10, 10' can be connected to a side edge of one of the radiation slots 1, 1' using, for instance, a through hole. Moreover, the dielectric material 4 filling the rectangular parallelepiped surrounded by the first and second sets of conductive plates can be replaced with air. Embodiment 5 Figs. 12(a) - 12(c) schematically illustrate a configuration of the fifth embodiment of the present invention, Fig. 12(a) being a perspective view, Fig. 12(b) a cross-sectional view taken along the line A-A and Fig. 12(c) a side elevation. In these figures, a center conductor 13 of the signal feeding connector 7 is divided into two conductors 12, 12' which are divided respectively into two conductors 12a, 12b; 12c, 12d. The conductors 12a, 12b are each connected to a side edge of a corresponding one of the radiation slots 1, 1' provided in a vertical arrangement on the grounded conductive plate 2, while the other conductors 12c, 12d are each connected to a side edge of a corresponding one of the radiation slots 1, 1' provided in a vertical arrangement on the grounded conductive plate 2'. A difference in length of the signal feeding lines for the adjacent radiation slots 1, 1; 1', 1' formed on the same conductive plate is an integer times the operation wavelength. Therefore, the adjacent radiation slots 1, 1 on the grounded conductive plate 2 are excited in the same phase while the radiation slots 1', 1' on the other grounded conductive plate 2' are excited out of phase. Therefore, the electromagnetic waves radiated from the radiation slots formed on the same grounded conductive plate are in the same phase in the horizontal plane, resulting in increase in gain in the horizontal plane. Moreover, since the radiation slots 1, 1 on the conductive plate 2 are excited out of phase with respect to the radiation slots 1', 1' on the conductive plate 2', the radiation field produced by these radiation slots 1, 1; 1', 1' become continuous in the horizontal plane and a horizontally polarized omnidirectional high-gain radiation pattern can be obtained in the horizonal plane. The beam width in the vertical plane can be adjusted by changing an interval between the vertically arranged radiation slots on the same conductive plate. The number of radiation slots formed on the same conductive plate is not limited to two and three or more radiation slots can be provided. The signal feeding line may be replaced with other lines such as a coaxial line. Embodiment 6 Figs. 13(a) - 13(c) schematically illustrate a configuration of the sixth embodiment of the present invention, Fig. 13(a) being a plan view, Fig. 13(b) a cross-sectional view taken along the line A-A and Fig. 13(c) a cross-sectional view taken along the line B-B. In these figures, the center conductor 13 of the signal feeding connector 7 is divided into and connected to the strip lines 5, 5'. These strip lines 5, 5' are then divided into two strip lines 5a, 5b; 5c, 5d. The strip lines 5a, 5d are connected to different side edges of the radiation slots 1, 1' provided on the conductive plate 2', while the other strip lines 5b, 5c are connected to the different side edges of the radiation slots 1, 1' provided on the conductive plate 2. In this case, a difference in length of the signal lines for the radiation slots formed on the same conductive plate is set to an odd number times a half of the wavelength. Therefore, the radiation slots 1, 1' on one conductive plate 2 are excited in the same phase, while the radiation slots on the other conductive plate are excited out of phase. Accordingly, for the same reason as the fifth embodiment, a horizontally polarized omnidirectional high-gain radiation pattern can be obtained. The beam width in the vertical plane can be adjusted by changing an interval of the vertically arranged radiation slots on the same conductive plate. The number of radiation slots formed on the same conductive plate is not limited to two and three or more radiation slots can be provided. The signal feeding line may be replaced with other lines such as a coaxial line. Embodiment 7 Figs. 14(a) - 14(b) schematically illustrate a configuration of the seventh embodiment of the present invention, Fig. 14(a) being a perspective view and Fig. 14(b) a cross-sectional view taken along the line A-A. This embodiment is different from the fifth embodiment in that a plurality of pins 14 for connecting the first set of grounded conductive plates 2, 2' are provided in the antenna. The operation is the same as that explained in regard to the fifth and sixth embodiments. In this case, the periphery of the radiation slots 1, 1' is surrounded by the conductive plates 2, 2' and this configuration can be considered as a waveguide and a waveguide mode can be excited therein. If the width of the conductive plates 2, 2' is determined to be a half of the wavelength or less, only the basic mode is propagated if no connecting pin 14 is provided in the waveguide. The radiation slots 1, 1, 1', 1' formed along the center of the conductive plates 2, 2' are inherently not excited, but these radiation slots are actually excited because the internal electromagnetic field is disturbed due to the existence of the internal feeding lines 12, 12'. However, since the radiation slots are excited in the waveguide mode in a phase difference different from the case where the radiation slots are excited with the feeding line, the amplitude and phase at the radiation slots are disturbed and any omnidirectional radiation pattern cannot be obtained. In order to solve the problem, any unwanted waveguide mode is suppressed by the pins 14 connecting the conductive plates 2, 2', thereby obtaining an omnidirectional radiation pattern. In the seventh embodiment, the pins 14 are used for suppressing unwanted mode, but conductive bars or plates can be used in place of the pins 14. Embodiment 8 Figs. 15(a) and 15(b) schematically illustrate a configuration of the eighth embodiment of the present invention, Fig. 15(a) being a perspective view and Fig. 15(b) a side elevation. In this embodiment horn-type metal conductors 15, 15' are coupled to upper and lower surfaces of the antenna apparatus of the first - seventh embodiments. In this embodiment, for the same reasons as explained for the first embodiment, a horizontally polarized wave is excited omnidirectionally. If only one radiation slots 1, 1' is formed on each of the conductive plates 2, 2' like the first embodiment, there is a limitation to a change in beam width in the elevating direction and it is difficult to obtain a high gain. Instead of vertically arranging a plurality of radiation slots on the conductive plates 2, 2' to narrow the beam width in elevation, this embodiment employs the horn-type conductors 15, 15' coupled to the upper and lower ends of the antenna apparatus described in the foregoing embodiments. The horn-type conductors 15, 15' operate in combination like a horn antenna. Since the gain of this antenna is determined by a size of the aperture of the horn, a higher gain can be obtained by enlarging the aperture of the horn. This means that a high gain can be obtained even if only one radiation slot is provided on each of the conductive plates 2, 2'. A slant angle a of the horn-type conductors 15, 15' with respect to the horizontal plane does not give any influence on an omnidirectional pattern in the horizontal plane. The beam width and gain in the vertical plane can be easily adjusted by changing the slant angle a. Embodiment 9 Figs. 16(a) - 16(c) schematically illustrate a configuration of the ninth embodiment of the present invention, Fig. 16(a) being a perspective view, Fig. 16(b) a cross section taken along the line A-A and Fig. 16(c) a side elevation. This embodiment provides a third set of conductive plates 16, 16' that electrically connect the first set of conductive plates 2, 2' of the antenna apparatus of the first embodiment. In principle, an omnidirectional radiation pattern can be obtained if a size of the conductive plates 2, 2' is infinite. Since the conductive plates 2, 2' are limited in size, however, a ripple is generated due to the interference of waves diffracted at the edge portions of the conductive plates 2, 2'. The generated ripple changes in the period of about one wavelength depending on the size of the conductive plates 2, 2'. Since the ripple can be minimized by changing the size of the conductor plates 2, 2', in this embodiment, the conductive plates 16, 16' are additionally provided to cover the opposing conductive plates 3, 3" of the antenna apparatus according to the first to seventh embodiments. The third set of conductive plates 16, 16', though shown in Fig. 16(b) to have a semi-circular cross-section in order to change the size of the conductive plates 2, 2', can be formed to have an elliptic or rectangular cross-section. Whether the spaces between the conductive plates 3, 3" and the third set of conductive plates 16, 16' are filled with a dielectric material or not is optional. Fig. 17 schematically illustrates a radome 28 having radiation slots 29,29', 29",... and which accommodates any one of the omnidirectional antennas 30 described in the foregoing embodiments. In general, if a radome is used to protect an antenna apparatus, the radiation pattern is influenced to a certain degree by the radome even if the radome is transparent to an electromagnetic wave. To solve this problem, the radome 28 comprises a cylindrical cover of a dielectric material and a conductive film formed on the inner surface of the cylindrical cover, radiation slots 29, 29', 29", ... being formed on the conductive film in order to reradiate the electromagnetic wave to obtain an omnidirectional radiation pattern. Since a plurality of radiation slots are provided in the circumferential direction of the radome 28, an omnidirectional radiation pattern can be obtained without any influence given by the radome 28. It is noted that, a plurality of radiation slots may be arranged along the longitudinal axis of the radome 28 and dipole antennas may be used in place of the slots. Fig. 18 schematically illustrates a configuration of a transponder comprising a transceiver, any one of the omnidirectional antenna apparatus 30 according to the present invention described heretofore, a transceiver 33, a battery 34 and the radome 28. The transponder comprises a switch 35, an indicator 36 for indicating that the transceiver 33 is waiting for a signal received, an indicator 37 for indicating that the transceiver 33 is transmitting a signal and an indicator 38 for indicating a level of received signal. The transponder can improve a man-machine relation within a limit of a predetermined volume and weight by utilizing the omnidirectional antenna which is designed smaller than a conventional waveguide slot antenna. This transponder makes particular contribution to the improvement in relation between an operator and the machine when emergent signal transmission is required. In order to prevent an operator who is to transmit an emergency signal from forgetting to turn ON the switch 35, the transponder is provided with the indicator 35 as a means for informing that the transceiver 33 can receive a signal and transmit a response, that is, that the transceiver has been activated and is waiting for reception of a signal. The transponder is provided with the indicator 37 as a means for informing an operator that the transceiver has been activated and is transmitting a signal, whereby the operator can confirm that the transponder is correctly operating. In addition, the transponder is provided with the indicator 38 as a means for enabling an operator to monitor a level of received signal, thereby confirming whether or not a searching plane is coming closer.

Claims: An antenna apparatus having radiation slots (1,1') arranged at opposite positions on a grounded conductive hollow body and a single signal feeding line (8) entering said antenna to excite said slots (1,1) with a signal, said grounded conductive hollow body comprising a set of parallel conductive plates (2, 2') each including one of said radiation slots (1, 1'), said feeding line (8) and slots (1, 1') being arranged to excite said slots (1, 1') out of phase to form an omnidirectional radiation pattern in a plane perpendicular to said hollow body, strip lines (5) being provided for connecting said signal feeding line (8) to said parallel conductive plates (2); characterised in that said strip lines (5) and parallel conductive plates (2) constitute a triplate line exciting said radiation slots. An antenna apparatus as recited in claim 1, wherein said hollow body is filled with a dielectric material (4). An antenna apparatus as recited in claim 2, comprising a through-hole (9) formed between said radiation slots (1, 1'). An antenna apparatus as recited in claim 1, 2 or 3, wherein said hollow body is a rectangular hollow body formed by said set of conductive plates (2, 2') being connected together by a second set of conductive plates (3, 3'), said radiation slots (1, 1') being formed in the opposing conductive plates (2). An antenna apparatus as recited in claim 4 wherein a plurality of radiation slots (1, 1') are provided along the longitudinal axis of said hollow body, said radiation slots (1, 1') formed on the opposing conductor plates (2) being excited out of phase and said radiation slots (1, 1') formed on the same conductor plate being excited in phase. An antenna apparatus as recited in claim 5, wherein a difference in length of signal feeding lines (12, 12') for feeding adjacent said radiations slots (1, 1') formed on the same conductive plate (2) is set to have an integer times an operating wavelength. An antenna apparatus as recited in claim 5, wherein a difference in length of signal feeding lines (5, 5') for feeding adjacent said radiation slots (1, 1') on the same conductive plate (2) is set to be an odd number times a half of an operating wavelength. An antenna apparatus as recited in any one of claims 3 to 7, wherein horn-type conductor plates (15, 15') are provided to the conductive plates (2) perpendicular to the longitudinal axis of said rectangular hollow body. An antenna apparatus as recited in any one of claims 3 to 7, wherein semi-cylindrical conductor plates (16, 16') are respectively mounted to the conductive plates (2) parallel to the longitudinal axis of said hollow body for the purpose of reducing any influence of waves diffracted at the edges of the conductive plates (2). An antenna apparatus as recited in any one of claims 3 to 9, comprising dielectric material layers (11, 11') formed on said opposing conductive plates (2, 2') and signal feeding lines (5, 5') provided on said dielectric material layers (11,11').

Inventors: KATAGI TAKASHI, OHMINE HIROYUKI, SATO SHIN-ICHI, SUNAHARA YONEHIKO, WADAKA SHUSOU, KATAGI, TAKASHI, OHMINE, HIROYUKI, SATO, SHIN-ICHI, SUNAHARA, YONEHIKO, WADAKA, SHUSOU, Katagi, Takashi Mitsubishi Denki Kabushiki Kaisha, Ohmine, Hiroyuki Mitsubishi Denki Kabushiki Kaisha, Sato, Shin-Ichi Mitsubishi Denki Kabushiki Kaisha, Sunahara, Yonehiko Mitsubishi Denki Kabushiki K., WADAKA, Shusou Mitsubishi Denki Kabushiki Kaisha
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1994
Main classification: H01Q13
IPC codes: H01Q21, H01Q13